1. Field of the Invention
The present invention relates to light emitting elements using electroluminescence. In addition, the present invention relates to a light emitting device and an electronic device having the light emitting element.
2. Description of the Related Art
In recent years, research and development has been extensively conducted on light emitting elements using electroluminescence. In a basic structure of these light emitting elements, a substance having a light emitting property is interposed between a pair of electrodes. By application of voltage to these elements, light emission can be obtained from the substance having a light emitting property.
Since such a light emitting element is of self-light emitting type, it is considered that the light emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and the like and is therefore suitable for a flat panel display element. Another major advantage of such a light emitting element is that it can be manufactured to be thin and lightweight. In addition, extremely high response speed is also a feature.
Since the light emitting element can be formed into a film shape, planar light emission can be easily obtained by forming a large-area element. This is a feature that is hard to be obtained in point sources typified by an incandescent lamp and an LED or linear sources typified by a fluorescent light. Therefore, the light emitting element has a high utility value as a surface light source that can be applied to lighting and the like.
Light emitting elements using electroluminescence are classified broadly according to whether they use an organic compound or an inorganic compound as a substance having a light emitting property.
When an organic compound is used as a substance having a light emitting property, electrons and holes are injected into a layer including an organic compound having a light emitting property from a pair of electrodes by voltage application to a light emitting element, so that current flows therethrough. Then, the carriers (electrons and holes) are recombined, and thus, the organic compound having a light emitting property is excited. The organic compound having a light emitting property returns to a ground state from the excited state, thereby emitting light. Owing to this mechanism, such a light emitting element is referred to as a current-excitation light emitting element.
Note that the excited state generated by an organic compound can be types of a singlet excited state and a triplet excited state, and light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
In improving the element characteristics of such a light emitting element, there are many problems caused by the material, and in order to solve such problems, an improvement of the element structure, a development of materials, and the like have been carried out.
For example, in Non-Patent Document 1, a light emitting element with high efficiency is realized by using a method called Triplet Harvesting.